Which substances conduct electricity?

Read our standard health & safety guidance

Lesson organisation

This works well as a class experiment, with students working in groups of two to three. There will not be time to investigate all the substances, so each group could be assigned three or four of these, and the results pooled at the end.

Apparatus and chemicals

Eye protection

Each working group requires:

Carbon (graphite) electrodes fitted in a holder (see note 1)
Bunsen burner
Tripod
Pipe-clay triangle
Heat resistant mat
Clamp and stand
Small pieces of emery paper
Connecting leads and crocodile clips
Six-volt DC power pack
Six-volt light bulb in holder (see note 2)

Small pieces of lead (Toxic), copper (Low hazard) and perhaps other metals.

Crucibles containing samples of:
Phenylsalicylate (salol) (Irritant, Dangerous for the environment)
Polythene
Wax (Low hazard)
Sugar (Low hazard)
Zinc chloride (Corrosive, Dangerous for the environment)
Potassium iodide (Low hazard)
Sulfur (Low hazard) (optional - see note 3)

Technical notes

Lead (Toxic) Refer to CLEAPSS Hazcard 56
Copper (Low hazard) Refer to CLEAPSS Hazcard 26
Phenylsalicylate (salol) (Irritant, Dangerous for the environment) Refer to CLEAPSS Hazcard 52
Wax (Low hazard) Refer to CLEAPSS Hazcard 45B
Sugar (Low hazard) Refer to CLEAPSS Hazcard 40C
Zinc chloride (Corrosive, Dangerous for the environment) Refer to CLEAPSS Hazcard 108A
Potassium iodide (Low hazard) Refer to CLEAPSS hazcard 47B
Sulfur (Low hazard) Refer to CLEAPSS Hazcard 96A
Sulfur dioxide (Toxic) Refer to CLEAPSS Hazcard 97

1 The carbon electrodes need to be fixed in some sort of support – such as a polythene holder or large rubber bung – so that there is no possibility of the electrodes being allowed to short-circuit. The electrodes need to be fixed in such a way as to fit inside the crucible supplied.

2 A light bulb has more visual impact, but an ammeter can be used instead.

3 Sulfur is a non-metallic element and is a good substance to have included in the list. But there is a strong likelihood of it catching fire, with sulfur dioxide (Toxic, Corrosive) given off. Sulfur fires are hard to extinguish. If it happens, cover the vessel with a damp cloth and leave in place until cool. If there is time, sulfur can be done as a teacher demonstration. Heat a small sample of ‘flowers of sulfur’ very, very slowly. Sulfur is a very poor conductor of heat, and localised heating is likely to cause it to start burning! You must use a fume cupboard.

Procedure

HEALTH & SAFETY: Wear eye protection

Part 1

a Set up the circuit as shown in the diagram, but at this stage do not include the crucible. You will need this later.

b Select one of the metals, and by holding the electrodes in contact with it, find out whether or not it conducts electricity.

c Note down the results using a table like the one you can download from here (148 KB), and repeat this experiment with each of the other metals available.

d Switch off the current if you have not already done so.

e Select one of the solids contained in a crucible. Lower the electrodes so that they are well immersed in the solid, and then clamp the electrodes in position.

f Switch on the current and find out whether the solid conducts electricity or not.

g Switch off the current.

h Set the crucible over a Bunsen burner on a pipe clay triangle and tripod, and clamp the electrodes in position over the crucible. Gently heat the sample until it just melts, and then turn off the Bunsen flame. If necessary lower the electrodes into the molten substance, before clamping them again.

i Switch on the current again. Does the molten substance conduct electricity now?

j Write up all your observations.

k Switch off the current, raise the electrodes from the crucible, and allow them to cool.

l Clean the electrodes with emery paper.

m Repeat steps e to l with some or all of the other solids.

n Pool your results with other groups so that your table is complete.

Teaching notes

The covalent solids only need to be heated for a short time for melting to take place. Under no circumstances should heating be prolonged, otherwise the substances may decompose and/or burn. The students should be warned about what to do if this happens eg cover with a damp cloth. The experiments should be done in a well-ventilated laboratory.

It may be helpful to reserve a crucible for each of the powdered compounds, while having one or two others that can be heated. Once a solid has been liquefied and allowed to cool, the solidified lump is often hard to break up or powder in the crucible.

Zinc chloride melts at about 285 °C, so heating needs to be fairly prolonged in comparison with the covalent solids. It will, however, produce chlorine (Toxic) so heating should stop as soon as conductivity is detected. Potassium iodide melts at about 675 °C, so very strong and prolonged heating is needed here.

Student questions and answers

Here are some questions for your students, with answers.

1 What do you conclude about the electrical conductivity of metals? (All the metals conduct electricity well. You should explain this conductivity in terms of the ‘free’ electrons within a metallic structure.)

2 Do any of the solid compounds conduct electricity? (No, none of them.)

3 Do any of the molten compounds conduct electricity. If so, which ones? (Yes, zinc chloride and potassium iodide.)

4 Why do some substances conduct only when they have been liquefied? (Some substances are ionic, but electrical conduction is only possible when the ions are free and mobile. This happens once the solid has been melted.)

5 Can you now classify all the compounds as being either ionic or covalent? (Phenylsalicylate, polythene, wax and sugar are covalent. Zinc chloride and potassium iodide are ionic.)

Health and Safety checked, February 2008